[Rd] certain pipe() use cases not working in r-devel

[Ivan Krylov]

В Wed, 14 Feb 2024 14:43:12 -0800
Jennifer Bryan <jenny.f.bryan using gmail.com> пишет:

> But in r-devel on macOS, this is silent no-op, i.e. "hello, world"
> does not print:
> 
> > R.version.string  
> [1] "R Under development (unstable) (2024-02-13 r85895)"
> > con <- pipe("cat")
> > writeLines("hello, world", con)  

I can reproduce this on 64-bit Linux.

I think that this boils down to problems with cleanup in R_pclose_pg
[*]. The FILE* fp corresponding to the child process pipe is created
using fdopen() in R_popen_pg(), but R_pclose_pg() only performs close()
on the file descriptor returned by fileno(). The FILE* itself is
leaked, and any buffered content waiting to be written out is lost.

One of the last few lines in the strace output before the process
terminates is the standard C library cleaning up the FILE* object and
trying to flush the buffer:

$ strace -f bin/R -q -s \
 -e 'writeLines("hello", x <- pipe("cat")); close(x)'
...skip...
write(5, "hello\n", 6)                  = -1 EBADF (Bad file descriptor)
exit_group(0)                           = ?
+++ exited with 0 +++

There is a comment saying "see timeout_wait for why not to use fclose",
which I think references a different function, R_pclose_timeout():

>> Do not use fclose, because on Solaris it sets errno to "Invalid
>> seek" when the pipe is already closed (e.g. because of timeout).
>> fclose would not return an error, but it would set errno and the
>> non-zero errno would then be reported by R's "system" function.

(There are no comments about fclose() in timeout_wait() itself.)

Is there a way to work around the errno problem without letting the
FILE* leak?

-- 
Best regards,
Ivan

[*] Introduced in https://bugs.r-project.org/show_bug.cgi?id=17764#c6
to run child processes in a separate process group, safe from
interrupts aimed at R.